1. Field of the Invention
This invention relates generally to a process of manufacturing the main body of a bike helmet and, more particularly, to one capable of lowering cost, boosting yield rate, solidifying the helmet and prolonging years of use.
2. Description of the Prior Art
The main body of a bike helmet is made up of a shock absorbing liner and a shell. The shock absorbing liner is made from thermoformed expanded polystyrene (EPS) material, and the shell is made from vacuum formed polycarbonate (PC) or polyvinyl chloride (PVC) sheets. The way of combining the EPS with the shell can be categorized into the following two kinds:    (I.) A two-piece unit made from the combination of the EPS and the shell, shown in FIG. 1, is a middle and low-price product, and its manufacturing process is as follows:            1.) Cut out an adequate size for a PVC or PC sheet, and print a pattern.        2.) Heat the PVC or PC sheet with the printed pattern to be softened, followed by an exploiting vacuum forming method to get a shell 2.        3.) Trim the edges and openings of the shell 2.        4.) Attach the shell 2 to the EPS 1 by a tape 11 (or glue) to form a head cover of a helmet.        
However, the two-piece category surely has the following shortcomings:                1.) During the process, the required molds, fixtures and cutters must consume extreme expenditure.        2.) Since the combination between the shell 2 and EPS 1 is adhered by some kind of adhesive, which couldn't be joined closely, bubbles and hollows exist in between. On the other hand, the tape 11 will sooner or later fall off, being subject to blowing wind, drying sun and sprinkling rain.        3.) Due to the constraint of the de-molding angle of the mold, the shell 2 fails to shield the completeness of the EPS 1. Therefore, this kind of helmet has a shorter life cycle, and its appearance is prone to cracks or depressions by accidental collisions.        4.) Trimming for irregular openings is always time consuming and leads to a high defective rate.            (II.) A single-piece unit made from all-in-one forming, and in mold forming (IMF), shown in FIGS. 2 & 3, is a high-end product, and its manufacturing process is as follows:            1.) Trim a PC sheet first, and print a pattern.        2.) Heat the PC sheet with the printed pattern to be softened, followed by an exploiting vacuum forming method to get a shell 2′.        3.) Trim the edges and openings of the shell 2′.        4.) Place the shell 2′ fixedly in the forming mold of the EPS 1′, filling in with the EPS substance, and heat the EPS particles with steam to expand into a forming head cover.        
However, this single-piece category again has the following shortcomings:                1.) The mold calls for high precision, which substantially boosts the cost of the molding.        2.) The printing ink for the print of the pattern must sustain high temperature and feature great ductility, which substantially boosts the cost of the printing ink.        3.) Trimming for irregular openings is always time consuming and leads to a high defective rate, which demands investment for automatic trimming equipment, and it boosts the cost of the investment.        4.) The tightness between the shell and the mold is highly demanded, the making control over it hard, and that lifts the defective rate.        